Copper-Doped Titanium Dioxide Bronze Nanowires with Superior

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Copper Doped Titanium Dioxide Bronze Nanowires with Superior High Rate Capability for Lithium Ion Batteries Yongquan Zhang, Yuan Meng, Kai Zhu, Hailong Qiu, Yanming Ju, Yu Gao, Fei Du, Bo Zou, Gang Chen, and Yingjin Wei ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b10766 • Publication Date (Web): 10 Mar 2016 Downloaded from http://pubs.acs.org on March 18, 2016

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ACS Applied Materials & Interfaces

Copper Doped Titanium Dioxide Bronze Nanowires with Superior High Rate Capability for Lithium Ion Batteries

Yongquan Zhanga,b, Yuan Menga, Kai Zhua, Hailong Qiua, Yanming Jua, Yu Gaoa, Fei Dua, Bo Zouc, Gang Chena,c, Yingjin Weia,*

a

Key Laboratory of Physics and Technology for Advanced Batteries (Ministry of

Education), College of Physics, Jilin University, Changchun 130012, P. R. China. b

College of Applied Science, Harbin University of Science and Technology, Harbin

150080, P. R. China. c

State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, P.

R. China.

*

Corresponding author: [email protected] (Y. J. Wei) Tel & Fax: 86-431-85155126

ACS Paragon Plus Environment


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Abstract Pristine and Cu-doped TiO2-B nanowires are synthesized by the microwave assisted hydrothermal method. The doped oxide exhibits a highly porous structure with specific surface area of 160.7 m2 g-1. As evidenced by X-ray photoelectron spectroscopy and X-ray energy dispersive spectroscopy, around 2.0 at.% Cu2+ cations are introduced into TiO2-B, which leads to not only a slightly expanded lattice network but more importantly a modified electronic structure. The band gap of TiO2-B is reduced from 2.94 eV to 2.55 eV resulting in enhanced electronic conductivity. Cyclic voltammetry and electrochemical impedance spectroscopy reveal improved electrochemical kinetic properties of TiO2-B due to the Cu doping. The doped nanowires show specific capacity of 186.8 mAh g-1 at the 10 C rate with capacity retention of 64.3 % after 2000 cycles. Remarkably, our material exhibits a specific capacity of 150 mAh g-1 at the 60 C rate, substantiating its superior high rate capability for rechargeable lithium batteries.

Keywords lithium ion batteries, anode material, titanium dioxide bronze, copper doping, electrochemical properties

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1. Introduction Titanium dioxides (TiO2) have been the subject of considerable attention in energy and environmental sciences particularly due to their wide uses for dye sensitized solar cells, photocatalysis and lithium ion batteries (LIBs). The applications of TiO2 in the first two technologies primarily derive from their unique electronic structure with a suitable band gap of 3.0 eV.1-2 As for LIBs, TiO2 has some prominent merits including good cycle stability, high rate capability, low price and environment-friendly. In addition, its relatively high charge-discharge potential can effectively depress the formation of Li dendrite and SEI film which significantly strengthens the battery safety.3-4 The occurrence of TiO2 in nature is known for the rich polymorphism including anatase, rutile and bronze (TiO2-B). Whereas anatase and rutile were mainly considered as promising materials for solar cells and photocatalysis, a growing number of studies have suggested that in the LIBs TiO2-B be a better choice than the other two polymorphs. The unique three dimensional framework of TiO2-B can accommodate more than 250 mAh g-1 of Li ions.5-6 Moreover, the pseudo capacitive Li+ storage in TiO2-B makes it possible to charge-discharge at ultra-high current rates.7 A major problem of TiO2, however, lies in its intrinsic low electronic conductivity that seriously hinders the high rate capacity of the material. Carbonaceous materials such as carbon nano tubes and reduced graphene oxides were used as conductive additives to improve the electronic conductivity of titanium dioxides.8, 9 But, these carbonaceous additives only enhance the external electronic conductivity of the material; the electronic conductivity of the material bulk is still low. Recently, both theoretical and experimental works show that the electronic structure of TiO2 could be engineered by some aliovalent transitional metal cations such as Nb5+, Zn2+, V5+, Fe3+10-13 and non-metal anions such as P5+, N3-, S2- and F-14-17. The dopant ions could narrow the band gap of TiO2 or form 3



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localized states in the band structure which help a lot for the photocatalysis properties of TiO2. The dopant ions also have a profound effect on the electronic conductivity of TiO2. A variety of dopant ions such as W6+, Nb5+, N3-, S2-, C and Sn4+ have shown the ability of improving the electrochemical performance of anatase attributed to the improved conductivity.18-23 But, due to the metastable property of TiO2-B, only a few attempts have been done to dope this material by cation or anion species. For example, R. Grosjean et al. showed that the 5 C rate discharge capacity of TiO2-B was increased from 130 to 170 mAh g-1 by Fe doping.24 In our recent work, we found that N3- ions have significant effect on the structure stability of TiO2-B but hardly affect the rate capability of the material.25 In order to find a more effective doping species, herein we prepared Cu-doped TiO2-B nanowires by the microwave assisted hydrothermal method. Then we compared their structure, physical and electrochemical properties with the un-doped TiO2-B nanowires. It shows that Cu2+ could be a suitable doping species for TiO2-B which not only improves the cycle stability but also significantly enhances the rate capability of the material.

2. Experimental For the preparation of Cu-doped TiO2-B nanowires, 0.5 g P25-TiO2 and a proper amount of Cu(NO3)2•3H2O were dissolved in 60 mL 10 M KOH solution. The suspension was shifted into a Teflon vessel and heated at 200 oC for 90 min under 500 W microwave irradiation. Then, the precipitate was collected and washed by water (step-1). The precipitate was dispersed into 0.1 M HNO3 for 4 h to extract the K+ ions via the ion-exchange reaction (step-2). The proton exchanged precipitate was mixed with 60 mL 0.1 M HNO3 solution and then transferred into the Teflon vessel, followed by a heating treatment at 180 oC for 90 min under microwave irradiation. The obtained powder was 4


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washed carefully and freeze-dried at -30 oC (step-3). Then, the material was post-treated at 400 oC for 2 h in Ar flow to get the final product (step-4). For comparison, pristine TiO2-B nanowires were prepared following the same procedures but without using Cu(NO3)2•3H2O. The crystal structure of the materials was studied by X-ray diffraction (XRD) on a Bruker AXS D8 diffractometer. Scanning electron microscope (SEM) was performed on a JSM-6700 field emission scanning electron microscope. High-resolution transmission electron microscope (HRTEM) was taken on FEI Tacnai G2 coupled with a BRUKER AXS X-ray energy dispersive spectroscopy (EDX). The EDX and elemental mapping analysis were performed using a Ni grid (200 mesh) as the sample holder. Raman scattering analysis was performed on a Thermo Scientific FT-Raman using a laser excitation of 532 nm. X-ray photoelectron spectroscopy (XPS) was collected on a VG Scientific ESCALAB spectrometer. UV-vis absorption spectrum was collected on a Perkin Elmer Lambda 950 UV-vis-NIR spectrophotometer. The specific surface area and pore size distribution of the materials were measured by nitrogen adsorption-desorption isotherms on a Micromeritics ASAP 2010 instrument. The 2032 coin cells with metallic Li as the anode were used for electrochemical experiments. The cathode was made by a slurry containg 70 % active material, 15 % super P and 15 % PVDF binder, using copper foil as the current collector. A piece of Celgard 2320 membrane was used as the seperator. The electrolyte was 1 mol L-1 lithium hexafluorophosphate (LiPF6) dissolved in a ethylene carbonate (EC) and dimethyl carbonate (DMC) mixed-solvent with volume ratio of 30 : 70. Galvonostatic cycling experiemnt was performed on a LAND-2010 automatice battery tester. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were collected on a VSP potentiostatic-galvanostatic system (Bio-Logic, France). 5



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3. Results and discussion The structure and chemical compositions of all intermediate products for the synthesis of TiO2-B nanowires are studied by XRD and EDX (Supporting Information, Figure S1). The EDX analysis results are provided in Table 1. Interpretation of the experimental results can be divided into four steps according to the preparation procedures: (step-1) after the initial hydrothermal reaction, the intermediate product shows a crystal structure as that of K2Ti8O17 (JCPDS no. 80-2023). The atomic ratio of K/Ti of the product is 24.98 %, which is in agreement with that of K2Ti8O17; (step-2) after the K+/H+ exchange in HNO3, the crystal structure of the material is similar as that of H2Ti8O17 (JCPDS no. 36-0656). The atomic ratio of K/Ti of the material is 5.34 %, indicating an incomplete K+/H+ exchange reaction; (step-3) after the hydrothermal reaction in HNO3, the material shows a XRD pattern as that of TiO2-B (JCPDS no. 74-1940). EDX analysis shows all the remaining K+ ions are removed from the material; (step-4) after post-annealing at 400 oC, the TiO2-B product shows higher crystallinity. But, a weak XRD peak appears at ~ 37.8o which is attributed to the (004) diffraction of anatase (JCPDS no. 21-1272). Figure S2 (Supporting Information) shows the XRD and EDX patterns of the intermediate products for the Cu-doped TiO2-B. The evolutions of crystal structure are the same as those of pristine TiO2-B. Moreover, a Cu/Ti atomic ratio of 2.55 % is obtained after the step-1, which indicates that Cu2+ ions are doped into K2Ti8O17. With the following reaction, the Cu/Ti atomic ratio decreases to 2.17 %, indicating that a part of Cu2+ ions are dissolved into the acid solution. After the step-3, all K+ ions are removed from the material. In the meanwhile, Cu2+ ions keep dissolving into the HNO3 solution during the hydrothermal process resulting in an even smaller Cu/Ti ratio of 1.67 %. Finally, only 1.62 % of Cu2+ ions are doped into TiO2-B after the last preparation step. In addition, an anatase impurity is also observed for the Cu-doped 6


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TiO2-B product. Table 1. EDX analysis results of the intermediate products for the preparation of pristine and Cu-doped TiO2-B nanowires. TiO2-B K/Ti (at.%)

Cu-doped TiO2-B

Cu/Ti (at.%)

K/Ti (at.%)

Cu/Ti (at.%)

Step-1

24.98

0

25.85

2.55

Step-2

5.34

0

5.22

2.17

Step-3

0

0

0

1.67

Step-4

0

0

0

1.62

The XRD patterns of the pristine and Cu-doped TiO2-B materials are displayed in Figure 1, together with the standard JCPDS cards of TiO2-B (no. 74-1940) and anatase (no. 21-1272). Both samples show the typical diffraction pattern of monoclinic TiO2-B except the (004) peak of anatase impurity. The broad XRD peaks could be due to the overlap of multiple diffractions, as well as the small particle size of the materials. TiO2-B has a three dimensional framework built by corrugated sheets of corner- and edge-shared TiO6 octahedral that are linked by bridging oxygens as shown in the inset of Figure 1.26 The lattice parameters of the pristine TiO2-B are determined as a = 12.345 Å, b = 3.764 Å, c = 6.452 Å, β = 108.21o and V = 284.79 Å3 which are changed to a = 12.328 Å, b = 3.789 Å, c = 6.587 Å, β = 107.90o and V = 292.78 Å3 after Cu doping. The non-negligible volume expansion of ~2.8% could be due to the larger ionic radii of Cu2+ (r = 73 pm) compared to that of Ti4+ (r = 61 pm),27 indicating that the Cu2+ ions are successfully doped into TiO2-B. According to Yang et al.,28 the weight ratio of TiO2-B to the anatase impurity could be determined by the intensity ratio of the peak at 33.4o to the one at 37.8o which correspond to the (-311) diffraction of TiO2-B and the (004) diffraction of anatase, respectively. Based on the present XRD data, the amounts of anatase impurity in the pristine and Cu-doped TiO2-B are calculated as 14.5 and 15.1 wt.% 7



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respectively.

Figure 1. XRD patterns of the materials together with the standard JCPDS cards of TiO2-B (no. 74-1940) and anatase (no. 21-1272), (inset): schematic view of the crystal structure of TiO2-B.

Figure 2 shows the Raman patterns of the samples. The pristine material shows a typical Raman pattern of monoclinic TiO2-B, except the weak peak at 514 cm-1 which is attributed to anatase.29 Anatase is a common impurity of TiO2-B products.30 According to Figure S1 and S2 (Supporting Information), the formation of this impurity could be due to partial phase transformation of TiO2-B during the post-annealing process, i.e. from step-3 to step-4. It has been shown recently that the anatase impurity could improve the electrochemical properties of TiO2-B due to the interphase interaction between the anatase and TiO2-B domains.31 According to this point-of-view, it is not mandatory to completely eradicate the anatase impurity in the products. No additional Raman band is observed for the Cu-doped material which indicates that the local 8


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structures of TiO2-B are not changed by Cu doping. It is noticed that the peak intensities for the Cu-doped material are considerably lower than those of the pristine TiO2-B. The typical peak for the anatase impurity could be only observed under an enlarged view (inset of Figure 2). Since the Raman experimental conditions (including the amount of sample, power of the laser excitation, and exposure time et al.) are almost the same for both samples, the different Raman intensities should be related with the intrinsic properties of the materials. One possible reason could be due to the smaller band gap of the Cu-doped material. As will be shown below, the band gap of TiO2-B is shortened from 2.94 eV to 2.55 eV by Cu doping. During Raman experiment, a part of incident photons with energy of hߥ are absorbed by the material causing excitation of some electrons from the valence band to the conduction band while the others will be scattered by the material. The Cu-doped TiO2-B with a narrower band gap could absorb more photons than the pristine one does. Thus only a few photons could be scattered resulting in lower Raman intensities.

Figure 2. Raman spectra of the materials, (inset): the enlarged view for the Cu-doped TiO2-B. 9



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Figure 3. SEM photographs of the pristine (a) and Cu-doped TiO2-B (b); TEM and HRTEM photographs of the pristine (c, e) and Cu-doped TiO2-B (d, f).

Figure 3a and 3b display the FESEM photographs of the pristine and Cu-doped TiO2-B samples, respectively. The materials show a flocculus feature which are composed of interlinked nanowires. Under a larger magnification (Supporting Information, Figure S3), one can see that the nanowires have high aspect ratio with width of 5 ~ 15 nm and length of several micrometers. The nanowires are further confirmed by TEM as shown in Figure 3c and 3d, respectively. The HRTEM image of the pristine material (Figure 3e) exhibits ambiguous lattice fringes. But, the fast Fourier transform (FFT) pattern clearly shows the (200) and (110) diffractions of TiO2-B. The Cu-doped material shows a similar HRTEM image as that of pristine TiO2-B (Figure 3f). However, its FFT pattern is composed of concentric circles indicating lower crystallinity of the material. Further, scanning transmission electron microscope (STEM) was 10


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performed on the Cu-doped TiO2-B. Figure 4 presents the elemental mappings of the material from which one can see that Cu is uniformly dispersed throughout the material. It should be noted that we used Ni-grid for EDX and elemental mapping analysis. Since the atomic weight of Cu (63.5) is close to that of Ni (58.7), a few Ni signals could be collected and mix with Cu. But these impurity signals would not affect the analysis result too much. Nitrogen adsorption-desorption isotherm measurements show that the Brunauer-Emmett-Teller (BET) surface area of the pristine TiO2-B is 154.8 m2 g-1, while the Cu-doped TiO2-B possesses a little bit larger surface area of 160.7 m2 g-1 (Figure S4, Supporting Information). The average pore size of the pristine and Cu-doped TiO2-B is 23 nm and 21 nm, respectively, indicating a highly porous structure of the materials. The high porosity could be originated from aggregation of the nanowries as detected by SEM and TEM.

Figure 4. Mappings of the Ti, O and Cu elements of the Cu-doped TiO2-B nanowires.

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Figure 5 shows the XPS patterns of the samples. The Ti 2p3/2 XPS of the Cu-doped TiO2-B is observed at 458.6 eV which is in agreement with that of Ti4+.32 This indicates that Cu doping does not affect the oxidation state of titanium. In comparison to the pristine material, observation of the Cu 2p peak for the Cu-doped TiO2-B confirms the successful Cu doping into the material. The Cu 2p3/2 and 2p1/2 XPS are observed at 932.6 and 952.1 eV, respectively, which are consistent with those of Cu2+.33 The atomic ratio of Cu/Ti is measured as 2.18 at.% which is close to that obtained by the EDX analysis. It is worth noting that substituting a Cu2+ cation at the Ti4+ site will give a net charge of -2 (CuTi'') which will be neutralized by an oxygen vacancy (VO··). Thus, as observed by HRTEM, the Cu-doped TiO2-B shows lower crystallinity than the pristine one due to the oxygen vacancies in the material. Figure 6 shows the UV-spectra of the materials. The reflectance measurement shows that the onset of the band edge absorption shifts from 422 nm to 486 nm after Cu doping. In addition, a broad band is observed at 600 ~ 900 nm which is assigned to the 2Eg to 2T2g transitions of Cu2+.34 The band gap of TiO2-B is calculated from the tangent lines in the plots of the square root of the Kubelka-Munk functions against the photon energy (Supporting Information, Figure S5).35 Analysis shows that the band gap of TiO2-B is reduced from 2.94 eV to 2.55 eV after Cu doping. According to the density functional theory (DFT) calculations, the Cu2+ dopants will generate localized Cu-3d defect states near the valence band maximum of TiO2 as shown in the inset of Figure 6.36 The electronic conductivity of the material is thus enhanced by the reduced band gap.

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Figure 5. Ti 2p (a), and Cu 2p (b) XPS of the pristine and Cu-doped TiO2-B nanowires.

Figure 6. UV-visible absorption spectra of the materails. (inset): band structures of the pristine and Cu-doped TiO2-B nanowires.

Figure 7 shows the cyclic voltammograms of the materials at different scan rates. 13



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According to Zukalová et al., the current peak “A” and “B” are due to Li+ insertion into TiO2-B and the peak “C” is attributed to the anatase impurity.29 Compared with XRD and Raman, CV analysis provides a more reliable way to discern the anatase impurity from the TiO2-B main phase due to the large difference of their Ti3+/Ti4+ redox potentials. Considering that the practical capacities of TiO2-B and anatase are 300 and 168 mAh g-1, respectively, the weight fractions of TiO2-B (W1) and anatase impurity (W2) could be calculated by the following relationship: CC 168W2 = C A + C B 300W1

(1)

Where Cc, CA and CB are the specific capacities of peak “C”, “A” and “B”, respectively, which are derived from CV analysis (Supporting Information, Figure S6). Based on this, the amounts of anatase impurity in the pristine and Cu-doped TiO2-B are calculated as 12.8 and 13.5 wt.%, respectively, which fit well with those obtained by XRD analysis. At each scan rate, the electrode polarizations of peak “A” and “B” are smaller than that of peak “C”. This indicates that the electrochemical kinetics of TiO2-B is better than that of anatase impurity. In addition, the electrode polarizations of the Cu-doped material are smaller than those of the pristine one indicating that Cu-doping improves the kinetic properties of TiO2-B. One can see that the current intensities of peak “A” and “B” show a linear relationship against v (inset of Figure 7), which is consistent with the pseudo capacitive property of TiO2-B. While the peak “C” displays a linear relationship against v1/2, indication of a traditional intercalation reaction.37 In addition, it is seen from Table 2 that the Cu-doped material always shows a larger slope than that of pristine TiO2-B which indicates that Cu doping improves the electrochemical kinetic properties of the material.

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Figure 7. Rate dependent CV curves of the samples, (inset): linear fitting of the current peaks . Table 2. Slopes of the linear fitting of current densities against scan rate. TiO2-B

S1a)

S2a)

Ab)

-1.07

-0.95

-0.52

Cu-doped TiO2-B -1.29 -1.36 b) Cathode current versus v; Cathode current versus v1/2.

-0.68

a)

Figure 8a shows the charge-discharge profiles of the materials collected at the 0.5 C rate. One can see a small inflection at ~ 1.7 V from the discharge profile. The process above this voltage is due to Li intercalation into the anatase impurity while the process below this voltage is attributed to the TiO2-B nanowires. The pristine TiO2-B shows initial discharge/charge capacities of 288.4/235.9 mAh g-1 with coulombic efficiency of 81.8 %. In comparison, the initial discharge/charge capacities for the Cu-doped material are increased to 294.8/251.7 mAh g-1 corresponding to a larger coulombic efficiency of 85.4 %. The coulombic efficiency increases to ~ 100 % after eight cycles highlighting the excellent electrochemical reversibility of the mateirals. The rate performances of the pristine and the Cu-doped TiO2-B nanowires are displayed in Figure 8b. The discharge capacities of the pristine TiO2-B at the 1, 10 and 30 C rates are 216.2, 170.9 and 128.2 mAh g-1, respectively, corresponding to 75.0 %, 59.3 % and 44.5 % of the first 15



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discharge. The material could deliver 103.6 mAh g-1 at the 60 C rate. But, the big fluctuations in specific capacities and coulombic efficiencies indicate that the electrode losses electrochemical stability at high current rates. In comparison, the Cu-doped nanowires show much improved rate capability especially above the 10 C rate. The discharge capacities at the 1, 10 and 30 C rates are 236.6, 180.2 and 156.6 mAh g-1, respectively. More excitingly, the material could still deliver 150 mAh g-1 at 60 C, which is 50.9 % of the first discharge. The Ragone plots of specific energy (E) and specific power (P) of the materials are calculated from the discharge profiles (Supporting Information, Figure S7). Apparently, the Cu-doped TiO2-B nanowires could deliver a maximum specific energy of 388.5 Wh kg-1 and a large specific power of 22.5 kW kg-1. In addtion, long term cycling of the materials was carried out at the 10 C rate as shown in Figure 8c. The discharge capacity of the pristine TiO2-B decreases from 185.8 to 89.2 mAh g-1 after 2000 cycles, resulting in capacity retention of 48 %. In comparison, the Cu-doped TiO2-B nanowires shows much improved cycle stability. A large specific capacity of 120.1 mAh g-1 could be obtained after 2000 cycles with capacity retention of 64.3 %. Detailed studies of the TiO2-B nanowires with different Cu contents are beyond the scope of this work. Here we only present some preliminary results on this aspect (Supporting Information, Figure S7). One can see that the current material with about 2.0 at.% Cu content shows the best electrochemical performance in all samples. However, intensive works are still needed to optimize the Cu content in TiO2-B. Comparing with the previous works on the Fe-doped and N-doped TiO2-B,24-25 this work shows that Cu2+ could be a more suitable doping species which not only improves the cycle stability but also significantly enhances the rate capability of TiO2-B.

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Figure 8. (a) Charge-discharge profiles; (b) rate capability; (c) cycling performance of the pristine and Cu-doped TiO2-B nanowires.

Electrochemical impedance spectroscopy of the materials was collected in the first discharge with a voltage interval of 0.2 V as shown in Figure 9. For both samples, the Nyquist plots obtained at 2.0, 1.8 and 1.6 V show an imperfect semicircle, while two well-resolved semicircles are obtained at 1.4, 1.2 and 1.0 V. The first semicircle is attributed to the SEI film and the second one is due to the charge transfer process. Reduction of electrolyte and formation of SEI film on TiO2-B have been studied by P. G. Bruce et al. The authors showed that these processes could even occur at the open-circuit voltage (OCV) and become much vigorous below 2.0 V.38 For the present Nyquist plots, the imperfect semicircles above 1.6 V could be caused by the similar time constants of the SEI film and the charge transfer process. Therefore, the obtained EIS data can be fitted by the equivalent circuit as shown in Figure 9 of which Rs is the ohmic resistance, Rf and Rct are the resistance of SEI film and charge transfer resistance, respectively, and 17



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W is due to the Warburg diffusion of lithium ions. The simulated EIS parameters are listed in Table 3. Both materials exhibit small ohmic resistance indicating that the battery cells are well prepared. In addition, the two materials have almost the same SEI resistance. This could be attributed to the similar microstructure of the samples such as dispersion of the nanowires, length and width of the nanowires and specific surface area of the materials et al. Under these conditions, the side reactions occurring in the materials are quite similar. The SEI resistance keeps rather stable during the whole discharge process. However, the SEI component diminishes rapidly when the electrode is subsequently charged to 3.0 V (Supporting Information, Figure S9). This indicates that the SEI film is partly removed on charging which is consistent with the previous FTIR and XPS analysis.38 The Rct parameter increases rapidly with the voltage decreasing from 2.0 V to 1.8 V and then increases slowly with further discharge. During the whole discharge process, the Cu-doped TiO2-B always shows a smaller Rct parameter than the un-doped material. This could be due to the larger electronic conductivity of the material which improves the charge transfer reactions of the battery cell.

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Figure 9. Nyquist plots of the pristine (a, c) and Cu-doped TiO2-B (b, d) nanowires at different discharging states. Table 3. Fitted EIS parameters of the pristine and Cu-doped TiO2-B nanowires. TiO2-B

Cu-doped TiO2-B

Voltage (V)

Rs (Ω)

Rf (Ω)

Rct (Ω)

Rs (Ω)

Rf (Ω)

Rct (Ω)

2.0

2.0

55.6

67.9

1.9

54.9

32.4

1.8

2.0

55.8

144.7

1.9

55.5

50.7

1.6

2.0

55.9

167.4

1.9

55.8

53.1

1.4

2.0

56.3

172.6

1.9

56.1

53.3

1.2

2.0

56.3

181.3

1.9

56.0

59.6

1.0

2.0

56.0

190.3

1.9

56.1

66.8

The chemical diffusion coefficients of lithium (DLi) could be determined by the following equation using the Warburg data,39

1  V DLi =  m 2  FSσ

  dE       dx  

2

(2)

Where Vm is the theoretical molar volume of TiO2-B (86.16 cm3 mol-1, the same data is used for the Cu-doped material, S is the electrode surface area (0.64 cm2), F the Faradaic constant, σ is the Warburg coefficient,

σ=

dZ ′

(3)

dω −1/2

which can be deduced from the linear fitting of Z′ vs. ω-1/2 (Supporting Information, Figure S10). The calculated chemical diffusion coefficients are displayed in Figure 10. One can see that the Cu-doped TiO2-B has larger chemical diffusion coefficients than the pristine one during the whole discharge. The chemical diffusion coefficient can be viewed as a product of self-diffusion coefficient DLi(self) and Wagner factor Φ,40 DLi = DLi(self) ⋅ Φ

(4)

DLi(self) is mainly related with the crystal structure of the material, and Φ is majorly 19



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determined by the concentrations and mobilities of Li+ and electrons. It generally reflects the influence of electrons motion on the diffusion of Li+ in internal electric filed. Based on the above analysis, the improved lithium diffusion coefficients of the Cu-doped TiO2-B could be related with the self-diffusion coefficient and the Wagner factor of the material. First, the Cu2+ cations expand the crystal lattice of TiO2-B to provide a more spaced structure for the diffusion of Li ions. In addition, it has shown that the defects (such as oxygen vacancies) in TiO2 could also facilitate the diffusion of Li ions.41 Moreover; the Cu2+ cations improve the intrinsic electronic conductivity of TiO2-B. Therefore, the diffusion of Li ions is facilitated under a stronger internal electric filed.

Figure 10. Lithium diffusion coefficients (DLi) of the pristine and Cu-doped TiO2-B nanowires as a function of discharge voltage.

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4. Conclusions To summarize, this work presents a microwave assisted hydrothermal method for the preparation of Cu-doped TiO2-B nanowires. The material possesses a highly porous structure and a large specific surface area. Cu doping serves to enhance the electronic conductivity of TiO2-B by shortening the band gap of the host material from 2.94 eV to 2.55 eV. These changes in the electronic structure lead to significantly improved electrochemical kinetic properties. According to the electrochemical characterizations, the Cu-doped TiO2-B exhibits superior electrochemical performance. The specific capacity is as high as 120.1 mAh g-1 at the 10 C rate and the capacity retention is 64.3 % after 2000 cycles. More significantly, a large specific capacity of 150 mAh g-1 could be obtained at an ultra-high charge-discharge rate of 60 C (I = 18 A g-1). These properties make it very promising as an anode material for high power lithium ion batteries.

ASSOCIATED CONTENT Supporting Information XRD and EDX patterns of the intermediate products for the pristine (Figure S1) and Cu-doped TiO2-B (Figure S2). SEM (Figure S3) and nitrogen adsorption-desorption isotherms (Figure S4) of the pristine and Cu-doped TiO2-B. The Tauc plots (ߙhߥ)1/2 vs (hߥ) of the pristine and Cu-doped TiO2-B (Figure S5). CV analysis (Figure S6), specific power vs. specific energy (Figure S7) of the pristine and Cu-doped TiO2-B, First charge-discharge curves and rate performance of the Cu-doped TiO2-B with different Cu contents (Figure S8), Nyquist plots (Figure S9) and linear fitting of the Z′ vs.ω-1/2 relationship (Figure S10) of the pristine and Cu-doped TiO2-B. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.XXX/XXXXXX. 21



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AUTHOR INFORMATION Corresponding Authors * Yingjin Wei, Email: [email protected] Notes The authors declare no competing financial interest.

Acknowledgements This work was supported by Ministry of Science and Technology of China (No. 2015CB251103), National Natural Science Foundation of China (No. 51472104, 21473075, 51272088), Jilin Provincial Science and Technology Department (No. 20140101083JC, 20150204078GX).

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Table of Contents Graphic

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Copper-Doped Titanium Dioxide Bronze Nanowires with Superior

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